1. Field of the Invention
The present invention relates to an image forming apparatus that includes a photosensitive member and an intermediate transfer member subjected to driving rotation control independently of each other, for rotation in contact with each, and performs control such that a difference in speed between the respective members at a contact portion is reduced to zero.
2. Description of the Related Art
Conventionally, an electrophotographic image forming apparatus, which is used as a copy machine or a multifunction peripheral, performs printing operations by forming an image for which a print command is received on a photosensitive drum as a toner image, transferring the toner image onto a recording sheet, and then fixing the toner image on the recording sheet. The electrophotographic image forming apparatus is provided with an intermediate transfer belt for having the toner image transferred thereto from the photosensitive drum and transferring the toner image onto a recording sheet.
Here, it is demanded that the photosensitive drum and the intermediate transfer belt are driven such that the speed of a surface on which the toner image is formed (hereafter referred to as “the surface speed”) is constant. In a case where the process speed of the photosensitive drum and that of the intermediate transfer belt are not constant, this causes image defects on an image transferred onto a recording sheet, which are called color shift (positional displacement between respective colors) and periodic positional displacement called banding. To overcome the above-mentioned problem, a motor as a drive source is subjected to speed feedback control using various speed detection sensors whereby the rotational speeds of the photosensitive drum and the intermediate transfer belt are controlled to respective target speeds. Note that as a drive motor, one employing a brushless DC motor is often used because of low-cost, quietness, and high efficiency. Further, for the speed feedback control using the brushless DC motor, a motor control method has been put into practical use which uses a rotary encoder disposed on a rotating shaft of the photosensitive drum and thereby controls the motor such that the rotating speed thereof becomes constant.
However, in the above-mentioned speed feedback control, only the rotational speed of the drum shaft is detected, but it does not directly detect the surface speed of the photosensitive drum. Therefore, it is difficult to control the surface speed of the photosensitive drum to a target speed e.g. due to an accuracy allowance of the diameter of the photosensitive drum and the like. The same problem occurs to the intermediate transfer belt due to an accuracy error of the diameter of a roller, variation in thickness of the intermediate transfer belt, and the like.
Further, aging and a change in the mechanical structure dependent on an operation environment are expected, and in view of transferability at a primary transfer section and interference of a drive system, a speed difference is sometimes provided between the surface speed of the photosensitive drum and that of the intermediate transfer belt. However, the speed difference is set to a value sufficiently small (e.g. 0.2%) with respect to the target process speed, and hence the set speed difference cannot be achieved due to the above-mentioned allowance in design of the mechanical structure and changes with time in the process speed. One method of avoiding this problem is to provide surface speed sensors for detecting surface speeds of the photosensitive drum and the intermediate transfer belt. However, from the viewpoint of cost and detection accuracy, it is difficult to actually implement the method.
To overcome this inconvenience, there has been proposed a method of deriving a rotational speed setting value of the rotary encoder when the speed difference between the surface speed of the photosensitive drum and that of the intermediate transfer belt becomes zero, without using any surface speed sensor (e.g. Japanese Patent Laid-Open Publication No. 2012-032515).
Technical points in such related art are shown in FIGS. 15A to 15C.
FIGS. 15A to 15C are diagrams useful in explaining related art for deriving a rotational speed setting value of the rotary encoder when the speed difference becomes zero, without using any surface speed sensor. FIG. 15A is a diagram showing changes with time of the target rotational speed value (VD) of the photosensitive drum, in a case where the intermediate transfer belt is drivingly controlled to a predetermined rotational speed (VITB—TAR), and the photosensitive drum is drivingly controlled while having the speed thereof varied within a speed range between a low speed value and a high speed value, including the predetermined rotational speed VITB—TAR. Note that the speed control of the photosensitive drum and the intermediate transfer belt is executed based on respective outputs from rotational speed sensors provided on rotating shafts thereof. Further, FIG. 15B is a diagram showing torque values TD produced by the photosensitive drum when the target rotational speed value is varied. FIG. 15C is a diagram showing degrees of change in the torque occurring when the target rotational speed value is varied.
FIG. 15C shows that when the surface speed of the photosensitive drum at the primary transfer section exceeds the surface speed of the intermediate transfer belt, the lord torque of the photosensitive drum suddenly changes. In other words, when the photosensitive drum is drivingly controlled to a target rotational speed value before or after which the degree of change in the torque value TD is large, the speed difference becomes equal to zero, and hence it is possible to set the target value of the surface speed in the image formation process to the target rotational speed value. Further, since the rotational speed setting values of the photosensitive drum and the intermediate transfer belt at which the speed difference becomes zero are derived, it is possible from this to set a desired speed difference.
As described above, in the method disclosed in Japanese Patent Laid-Open Publication No. 2012-032515, a rotational speed setting value before and after which the degree of change in torque occurring to the photosensitive drum becomes maximum is set as the target rotational speed value at which the speed difference becomes zero. According to this method, however, when the intermediate transfer belt is made of a resilient material or a like other adherent material, the friction coefficient varies with the speed difference, and particularly in a region where the speed difference is zero or in its vicinity, there occurs a phenomenon that the intermediate transfer belt repeatedly slips and tacks. Normally, in a case where the photosensitive drum is slower than the intermediate transfer belt, the intermediate transfer belt pulls the photosensitive drum by friction torque at the primary transfer section, whereas in a case where the photosensitive drum is faster than the intermediate transfer belt, the photosensitive drum pulls the intermediate transfer belt at the primary transfer section. Therefore, in the state of the speed difference being zero, it is expected that the torque caused by the photosensitive drum shows a largest degree of change. However, if the above-mentioned phenomenon occurs, the torque caused by the photosensitive drum drastically changes also at a point where the friction coefficient at the primary transfer section changes, and hence the rotational speed setting value before and after which the degree of change in the torque is maximum does not necessarily indicate the state of the circumferential speed being zero. Therefore, this method has a problem that the accuracy of deriving the point at which the speed difference becomes zero is not high.